1. Field of the Invention
The present disclosure is generally related to gypsum-based and calcium sulphate-based compositions and methods of making the same and, more particularly, is related to construction-grade gypsum-type compositions and methods of making the same.
2. Description of Related Art
Gypsum and calcium sulphate-based compositions and compounds are used in a variety of industries, particularly in the construction industry. For example, gypsum plaster is widely used in construction products such as self-levelers, such as in poured floor technology and repair mortars. Using heat to drive water from gypsum, or calcium sulfate dihydrate (CaSO4·2H2O), to form calcium sulfate hemihydrate (CaSO4·½H2O) generally produces gypsum plaster used in these products. The gypsum plaster is also referred to as plaster of Paris or stucco.
There are a number of processes used to perform the dehydration, or calcination, process. Calcination may be performed by, for example, flash drying at high temperature, cooking in large kettles, heating in furnaces or rotary kilns, using steam, or cooking in aqueous suspensions.
These many different techniques can result in plasters with a wide range of composition and properties, but generally two types are formed: alpha-hemihydrate type and beta-hemihydrate type. These two types are distinguished from one another by the amount of water that is necessary to make a pourable slurry with the finely ground powdered hemihydrate, with alpha-hemihydrate requiring less than about 50 mL per 100 g of plaster, and beta-hemihydrate requiring substantially above this amount, normally more than 70 mL per 100 g of hemihydrate plaster. This amount of water is known as the water demand.
A process is known for making alpha calcium sulfate hemihydrate suitable for a construction material from a moist finely divided gypsum obtained by desulfurization of flue gas from a power plant fired by brown coal or lignite, especially from a gypsum obtained by flue gas desulfurization from a wet flue gas desulfurization unit (called desulfogypsum or DSG). This process proceeds by recrystallization transformation of the calcium sulfate dihydrate contained in the DSG in the presence of saturated steam.
Different processes are known for transforming calcium sulfate dihydrate into alpha calcium sulfate hemihydrate. One such process for making the alpha-hemihydrate from natural gypsum is described in Ullmans Encyclopedia of Industrial Chemistry, 12, 301 (1976). In this process, calcium sulfate dihydrate pieces, namely naturally-occurring gypsum pieces, are fed to an autoclave and are converted to alpha-hemihydrate pieces in the autoclave in the presence of saturated steam at a temperature of 266° F. to 275° F. This alpha-hemihydrate product is dried above the temperature that hemihydrate will convert back to calcium sulfate dihydrate (˜113° F.) and is ground up for further use.
More specifically, the gypsum removed from a natural deposit is broken up into a grain size of 150 to 300 millimeters (mm), is filled into baskets, and is fed to an autoclave in the baskets. The autoclave is directly or indirectly heated with steam from 266° F. to 275° F. The heating is controlled so that a pressure of 4 to 5 bar (0.4 to 0.5 MPa) builds up in about four hours according to a saturated steam curve. Transformation of the calcium sulfate dihydrate to alpha-hemihydrate by this process usually takes at least six hours. The autoclave is then emptied.
The alpha-hemihydrate gypsum is introduced into a drying chamber in the baskets and dried under standard pressure at about 221° F. and subsequently finely ground. In the surface regions of the pieces of material, well-defined alpha-hemihydrate crystals grow in a more or less needlelike shape. FIG. 1 shows a scanning electron microscope (SEM) micrograph of an example of the needlelike crystals of alpha-hemihydrate obtained by this process.
Additives for control of the pH-value and for changing the crystal pattern can be metered into the autoclave and a product alpha-hemihydrate with various properties is obtainable. In this known process, however, the expensive purification steps are troublesome. In this process, distinct alpha-hemihydrate crystals arise more by chance, and control of the process in regard to crystal pattern and surface fine structure of the crystals formed is not provided.
In attempting to solve this problem, one process described in, for example, U.S. Pat. No. 5,015,449 issued to Koslowski, forms moist fine grained gypsum (calcium sulfate dihydrate) into a molded body at a pressure between 0.1 to 14 N/mm2 (MPa). Koslowski states that when forming a molded body by pressing the calcium sulfate dihydrate at pressures greater than 16 N/mm2, “one of course obtains molded or formed bodies but they are not autoclavable without forming fractures or cracks and are destroyed during autoclaving.” Koslowski at column 7, lines 41-45.
The molded gypsum body of the process disclosed in Koslowski has a total volume of 15 to 60% by volume pore volume, with more than 5% by volume of the pore volume containing air. When the starting material is wet, the remaining balance of the pores is filled with water. The molded body is then fed into an autoclave. The crystal growth and crystal pattern of the alpha-hemihydrate is regulated by control of a process temperature in the range between 230° F. and 356° F. and by pressure of the process atmosphere in the autoclave. The molded body is removed from the autoclave and delivered for use after the recrystallization transformation.
Prismatic columnar alpha calcium sulfate hemihydrate crystals are produced from this process, a SEM micrograph of which is shown in FIG. 2. The calcination time for the process for producing these crystals is approximately four to seven hours per batch. This long cycle time makes this process difficult and expensive from a production efficiency standpoint.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.